Sen. Claire McCaskill, D-Mo., has joined a growing list of politicians who now support same-sex marriage.

"I have come to the conclusion that our government should not limit the right to marry based on who you love," McCaskill posted Sunday on her Tumblr page. "While churches should never be required to conduct marriages outside of their religious beliefs, neither should the government tell people who they have a right to marry."

McCaskill's announcement in support of marriage equality comes as the U.S. Supreme Court takes up the controversial issue in two court cases this week. The justices will hear arguments Tuesday challenging California's Proposition 8. Then on Wednesday the high court takes up the federal Defense of Marriage Act.

Last year, when President Obama came out in support of same-sex marriage, a McCaskill spokesman told the Springfield News-Leader that the senator believes states "should take the lead in determining marriage equality." This is the first time the senator, a moderate Democrat, has publicly stated her support for gay marriage.

In her Tumblr post, McCaskill acknowledged that her views on same-sex marriage "have changed over time."

"But as many of my gay and lesbian friends, colleagues and staff embrace long-term committed relationships, I find myself unable to look them in the eye without honestly confronting this uncomfortable inequality," she wrote. "Supporting marriage equality for gay and lesbian couples is simply the right thing to do for our country, a country," which she said was founded on the principles "of liberty and equality."

McCaskill's announcement comes as prominent Democrats and Republicans have publicly announced their support for gay marriage. They include former secretary of State Hillary Rodham Clinton, whose husband signed into law the Defense of Marriage Act, and Republican Sen. Rob Portman of Ohio, who said his views changed because son Will came out and said he is gay.

In the past, McCaskill had expressed support for civil unions to same-sex couples. She was opposed to Missouri's 2004 constitutional amendment that banned gay and lesbian couples from marrying, and said it was unnecessary because existing Missouri law already prohibited gay marriage.

McCaskill, a moderate Democrat, won her second term in the U.S. Senate last year by defeating Republican Todd Akin, who garnered negative headlines for his controversial comments about "legitimate rape."

Portman's change of heart on gay marriage created a stir, in part because he is a prominent Republican. Portman said he told Mitt Romney's campaign that his son is gay as he was being vetted to be the GOP vice presidential candidate. Romney aide Beth Myers told NBC News Portman's son "did not play a role" in Romney's choice of a running mate.

In an op-ed for the Yale Daily News published Monday, Will Portman saluted his father and the senator's evolution on the subject of gay marriage.

"I'm proud of my dad, not necessarily because of where he is now on marriage equality (although I'm pretty psyched about that), but because he's been thoughtful and open-minded in how he's approached the issue, and because he's show that he's willing to take a political risk in order to take a principled stand," wrote the younger Portman, 21, a junior at Yale.

McCaskill closed her Tumblr post by acknowledging the divide on the issue.

"Good people disagree with me," the Missouri senator wrote. "On the other hand, my children have a hard time understanding why this is even controversial. I think history will agree with my children."